High intensity discharge (HID) lamps containing metal halide salts are widely used for lighting applications. For many years, the arc tubes used to contain the discharge were made of quartz. More recently, lighting manufacturers have introduced ceramic arc tubes for metal halide HID lamps. The arc tubes are made of polycrystalline alumina and are capable of operating at higher temperatures than quartz. Lamps with ceramic arc tubes exhibit reduced color shift over the life of the lamp, have improved efficacy and lumen maintenance, and have higher CRI values than similar lamps made with quartz arc tubes.
Initially, the ceramic arc tubes of commercial metal halide lamps had the shape of a right circular cylinder. These arc tubes typically were constructed of three to five separate pre-sintered ceramic parts such as described in U.S. Pat. No. 5,424,609. The parts were joined by interference fits in multiple sintering steps.
A more sophisticated arc tube design has been recently introduced which uses a two-piece arc tube construction. Such a construction is illustrated in cross-section in FIG. 1 and described in co-pending U.S. patent application Ser. No. 10/077,504, filed Feb. 15, 2000, now U.S. Pat. No. 6,620,272 which is incorporated herein by reference. This design represents a significant improvement over the prior three- and five-part constructions. Unlike the right-cylinder shapes of the prior constructions, this design has curved walls 19 in the end wells 17a, 17b and is commonly referred to as a bulgy shape. The bulgy shape provides a more uniform temperature distribution and a reduced hot spot temperature compared to right-cylinder shapes.
According to the method described in co-pending U.S. patent application Ser. No. 10/077,504, the arc tube is made by joining two molded ceramic halves in their green state. Heat is applied to the surfaces to be joined to cause a localized melting of the binder. The surfaces are then brought together and joined by alternately applying compression and stretching. As illustrated in FIG. 1, this method leaves a cosmetic seam 5 in the center of the arc tube where the two halves were mated.
The discharge chamber 12 of the arc tube contains metal halides salts, mercury, and a buffer gas. When the lamp is in operation, the metal halide salts form a molten condensate. The position of the metal halide salt condensate in the arc tube influences the spectral characteristics of the lamp. For vertically-operated arc tubes, the metal halide condensate resides generally in a pool 7 in the lower end well 17a. However, for reasons which are not completely understood, droplets 8 of the metal halide condensate will migrate up the inner wall of the arc tube to near the seam 5 in the center of the discharge chamber 12.
The migration of the condensate during lamp operation leads to an undesirable fluctuation in the color temperature of the lamps. This is because the color temperature of the emission from the arc discharge is higher when the condensate is located in the end well than when it is located in the center region of the arc tube. For example, the color temperature of a lamp containing a fill chemistry designed to operate at 4200K (10 wt. % NaI, 12 wt. % TlI, 33 wt. % CaI2, 15 wt. % DyI3, 15 wt. % HoI3, 15 wt. % TmI3) ranges from 4100 to 4400K when the condensate is located in the end well and from 3800 to 4000K when the condensate is located in within the central region of the discharge chamber. Therefore, in order to maintain a consistent color temperature, it is important to control the position of the condensate with the arc tube.